Is it best to exercise before or after breakfast for fat burning?

A decreased 24-h fat to carbohydrate ratio is one of the metabolic characteristics of subjects pre-disposed to obesity (Zurlo et al. 1990; Barwell et al. 2009). Glycogen has been shown to reach its lowest levels after an overnight fast and can be further decreased after an exercise bout.

In a recent study by Iwayama et al. (2015), exercise was performed at a workload of 50% VO2 max after breakfast, lunch, and dinner or without an exercise session (control). Carbohydrate depletion after the morning exercise session was equivalent to 18% of whole body glycogen storage (~2300kcal). Also, the lowest relative value of carbohydrate balance was found to be negatively correlated with fat oxidation. Interestingly, accumulated protein oxidation over 24-h were not different among each of the 4 trials.

This study represents the beneficial long-term effects of exercise in a carbohydrate-depleted state on glucose tolerance, insulin sensitivity, endurance capacity and fat oxidation have been reported (Van Proeyen et al. 2010; Hansen et al. 2005; Yeo et al. 2008). Also, although exercising whilst fasting, protein breakdown was no different between trials.

In conclusion, 24-h fat oxidation is increased by exercise only when performed before breakfast under energy balanced conditions.

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